pcpu: Remove unused definitions of ALT_STACK_SIZE

This was added originally for the sparc64 port and apparently copied to
other platforms.  No functional change intended.

MFC after:	1 week

sys/arm/include/pcpu.h

@@ -37,8 +37,6 @@
 #include <sys/_lock.h>
 #include <sys/_mutex.h>
 
-#define	ALT_STACK_SIZE	128
-
 struct vmspace;
 
 #endif	/* _KERNEL */

sys/arm64/include/pcpu.h

@@ -37,8 +37,6 @@
 #include <machine/cpu.h>
 #include <machine/cpufunc.h>
 
-#define	ALT_STACK_SIZE	128
-
 typedef int (*pcpu_bp_harden)(void);
 typedef int (*pcpu_ssbd)(int);
 struct debug_monitor_state;

sys/riscv/include/pcpu.h

@@ -42,8 +42,6 @@
 #include <machine/cpu.h>
 #include <machine/cpufunc.h>
 
-#define	ALT_STACK_SIZE	128
-
 /* Keep in sync with db_show_mdpcpu() */
 #define	PCPU_MD_FIELDS							\
 	struct pmap *pc_curpmap;	/* Currently active pmap */	\